1. Technical Field
The present invention relates to a metal-air fuel cell based on a solid oxide electrolyte employing metal nanoparticles as fuel.
2. Description of the Related Art
A fuel cell is an electrochemical device that generates electric energy based on change in free energy of fuel caused by oxidation of the fuel. Currently, various types of fuel cells, such as a phosphoric acid fuel cell, a polymer electrolyte membrane fuel cell, a molten carbonate fuel cell, a metal-air fuel cell, a solid oxide fuel cell, and the like, are used in the art.
Among these, a metal-air fuel cell is named as such because reactants employed therein include air or oxygen and a metal. In the metal-air fuel cell, air or oxygen injected from a cathode is ionized while passing through a cathode catalyst, and oxidizes the metal in an anode, thereby generating electrons. Thus, since the metal-air fuel cell can be operated simply by filling an anode material, this type of fuel cell has higher energy density than other types of fuel cell systems, can be operated at room temperature to be used for various application fields, and does not generate by-products other than metal oxides to achieve eco-friendly production of electric power.
However, the air-metal fuel cell has a short lifespan due to loss of a liquid electrolyte and evaporation of water resulting from air injection and oxidation. To solve this problem, a shut-off device is provided to the fuel cell to supplement water or an electrolyte. In this case, however, a user is exposed to a danger of directly handling a strongly basic liquid electrolyte. In addition, some components of the fuel cell can be corroded by the strongly basic liquid electrolyte.
On the other hand, a solid oxide fuel cell (SOFC) is one classified according to properties of electrolyte and accelerates reaction inside an anode under high temperature conditions of 600° C. to 1000° C. Thus, unlike the phosphoric acid fuel cell or the polymer electrolyte membrane fuel cell, the solid oxide fuel cell can provide very high energy conversion efficiency without using a platinum catalyst, which is an expensive material, and is not significantly influenced by reactivity of substances, thereby enabling use of various materials such as natural gas, coal gas, and the like, in addition to hydrogen. Further, the solid oxide fuel cell does not suffer from problems relating to corrosion and loss of an electrolyte, and thus can overcome the problems caused by the liquid electrolyte of the metal-air fuel cell.
Such a solid oxide fuel cell is composed of unit cells each including an electrolyte having high oxygen ion conductivity, and a cathode and an anode formed on opposite sides of the electrolyte. As fuels and raw materials for the solid oxide fuel cell, various materials may be used. For example, in the metal-air fuel cell system, oxygen ions are generated by reduction of oxygen supplied into the cathode, and moved to an anode catalyst through a solid electrolyte to react with a metal provided as a fuel, thereby producing metal oxides and electrons. Except for the use of the solid electrolyte as the electrolyte, this type of solid oxide-based metal-air fuel cell has a power generation mechanism of a typical metal-air fuel cell, and reactions in the cathode, solid electrolyte and anode of the fuel cell can be generally expressed as follows:Cathode: O2+4e−→2O2−Solid electrolyte: 2O2− movementAnode: Metal+2O2−→Metal oxide+4e−Overall reaction: Metal+O2→Metal oxide
Not only does such a solid oxide-based metal-air fuel cell solve the problem of the conventional metal-air fuel cell suffering from loss of the liquid electrolyte, but also allows application of inherent merits of the solid electrolyte fuel cell which promotes oxidation under high temperature operation conditions. However, as a major factor deteriorating power generation efficiency, the solid oxide-based metal-fuel cell suffers from high resistance due to a low degree of contact between the metal fuel and the catalyst, and there is a need for fuel materials having various physical chemical characteristics to solve such problems.